The goal of this project is to determine the number and kinds of cells with HIV-1, the level of maternal proviral and viral burden and maternal-fetal cellular transfer in mothers who transmit the virus to their infants. The author also will seek to determine the relationship between the genetically divergent genomes in maternal and infant blood, vaginal secretions, cord blood and their biological properties. The investigator hypothesizes that a genetically-defined subset of maternal viruses capable of entering the fetal circulation can be identified and that these viruses have escaped a critical component of the maternal immune surveillance mechanism. Additionally, he also hypothesizes that increased maternal viral burden from various body compartments is associated with an increased rate of transmission. To test the hypotheses about the potential mechanisms of perinatal HIV-1 transmission, as well as to provide important information about HIV-1 pathogenesis, the author plans to investigate the following three specific aims. First, he will attempt to assess whether or not a higher maternal viral burden, or a greater number of HIV-1-infected cells in the mother's blood, correlates with a higher rate of perinatal HIV-1 transmission using competitive reverse transcriptase (RT)- polymerase chain reaction (PCR), quantitative virus culture and PCR-driven in situ hybridization with flow cytometry. Next, he will try to define the evolutionary time course with which genetic variants arise in maternal blood and vaginal secretions during the course of infection seeking to uncover the origin of variants in the infant. Third, he will attempt to determine if maternal cells transferred across the placenta are responsible for perinatal HIV-1 transmission using PCR-driven in situ hybridization and flow cytometry with genetic analysis.